Updated 12:35 p.m. | The House chamber was closed Monday morning after Architect of the Capitol workers discovered a potentially hazardous substance during ongoing restoration work.

Behind wall fabric, workers found an unknown material that is being tested for asbestos. The chamber was closed "in an abundance of caution," House Sergeant-at-Arms Paul D. Irving said in a brief memo. Testing can take up to eight hours. The material was not airborne, according to a Capitol official. The discovery is not unusual for the type of renovation work or the age of the chamber, which was first used in December 1857.

The AOC did not provide any additional information, but updates are expected.

The hallways surrounding the House chamber were quiet Monday afternoon, with ropes blocking the usual entrances on the third floor where visitors can enter the gallery. Capitol Police officers were spotted sitting outside the Speaker's Lobby on the second floor, which had its doors closed and lights off.

The Capitol Visitor Center also stopped issuing passes for visitors to enter the gallery. A sign posted outside the office noted that the House gallery was closed.

After an asbestos emergency temporarily closed the House side of the Capitol in July 2014, union officials representing AOC workers told CQ Roll Call they were concerned about potential exposure to the human carcinogen, which can cause chronic lung disease as well as cancer.

The Office of Compliance, an agency created by Congress to ensure safety in the legislative branch workplace, subsequently inspected the incident for an alleged violation of the Occupational Safety and Health Act, and issued a report on May 21.

Both cases remain open, according to OOC General Counsel Amy Dunning. They will remain open until the findings, which are not publicly available, are abated.